The present invention relates to a system for restoring multi level/phase modulation data for use in a digital transmission system, and more particularly, to a method and apparatus for restoring multi level/phase modulation data to further improve reliability of data reconstruction by correcting a phase error which is produced in demodulation data.
Optimally, a transmission system should transmit at a predetermined transmission velocity with good reliability even if the transmission channel is in poor condition. This reliability is provided by using an error correcting coding/decoding method, such as a Trellis coded modulation (hereinafter, abbreviated as TCM) method. The TCM method combines data modulation with an error correction codification and improves the coding gain as the bandwidth of the transmission data is increased. Since the TCM technology is already known in the digital communication field, a detailed description thereof will be omitted. To improve the data transmission capability, an interleaving and deinterleaving method is also used in the digital transmission system.
FIG. 1 shows an example of a conventional apparatus for restoring multi level/phase modulation data. FIG. 1 shows a circuit for receiving data transmitted thereto. The data is assumed to have been interleaved, RS coded and Trellis coded, modulated and transmitted to a receiver. At the receiver the received data is demodulated and restored to its original data form.
In FIG. 1, I-axis data, which is in-phase data demodulated in a demodulator (not shown), and Q-axis data, which is quadrature-phase data demodulated in a demodulator (not shown), are input to input ports 11 and 12, respectively. Since the respective I-axis and Q-axis data have been interleaved, a deinterleaver 13 deinterleaves the respective I-axis and Q-axis data based on a predetermined sync position and outputs the deinterleaved data. The I-axis and Q-axis data appearing at the output of deinterleaver 13 is Trellis-decoded in a Trellis decoder 14. The Trellis decoder 14 determines whether or not the Trellis-decoded data is synchronized. A Viterbi algorithm is used for such a sync determination. The Trellis decoder 14 repetitively performs a process of selecting a path where the Hamming distance is shortest among paths of the data inputted to the respective state in a Trellis diagram, to find a path having the minimum Hamming distance entirely. If the cumulative distance of the found path is larger than a predetermined reference value, Trellis decoder 14 determines that the sync does not match and generates a predetermined out-of-sync signal S.sub.1. The predetermined reference value is determined by a designer and is experimentally obtained. Out-of-sync signal S.sub.1 at the output of Trellis decoder 14 is supplied to a first sync detector 15. First sync detector 15 generates a predetermined control signal S.sub.2, which is supplied to deinterleaver 13 whenever out-of-sync signal S.sub.1 is generated. Control signal S.sub.2 causes adjustment of the sync position for the deinterleaving operation of deinterleaver 13. Deinterleaver 13 slides the sync position whenever the sync adjustment control signal S.sub.2 is applied thereto, and performs a deinterleaving operation at the slide sync position. The data deinterleaved according to the slid sync position is again supplied to Trellis decoder 14. Trellis decoder 14 Trellis-decodes the input data again and once again determines if the sync is matched. Trellis decoder 14 finds a sync from the input data by repetitively performing such a set of sync detection processes.
If a sync condition is detected, the data decoded in Trellis decoder 14 is supplied to a block deinterleaver 16. Block deinterleaver 16 again deinterleaves, in units of a block, the data which has been interleaved in units of a block in a modulator at the transmission end. A second sync detector 17 detects a sync condition of the block data which is deinterleaved in block deinterleaver 16. Second sync detector 17 is disabled by out-of-sync signal S.sub.1 from Trellis decoder 14. The block data appearing at the output of block deinterleaver 16 is supplied to a Reed Solomon (RS) decoder 18. RS decoder 18 RS-decodes the data which has been RS-coded at the transmission end to reinforce an error correction function of the transmission data. Here, RS decoder 18 decodes the block data while matching a block sync signal S.sub.3 which is supplied from second sync detector 17.
Such a conventional restoring apparatus for multi level/phase modulation data does not restore exactly the original information data when a phase error occurs in the demodulated I- and Q-channel data.
To solve the problem of a phase error, a differential encoding/decoding method is currently used. The differential encoding method does not transmit data corresponding to the absolute value of the phase, but transmits only data corresponding to phase difference. The differential encoding/decoding method will be described below, with reference to 32-tuple quadrature amplitude modulation (QAM) data, which is Trellis-coded according to constellation of FIG. 2.
A codeword used in the 32-tuple QAM data, which is Trellis-encoded according to the FIG. 2 constellation, is a 5-bit (Y.sub.4 Y.sub.3 Y.sub.2 Y.sub.1 Y.sub.0) data codeword. Of these bits, the upper three bits (Y.sub.4 Y.sub.3 Y.sub.2) are uncoded bits, and the lower two bits (Y.sub.1 Y.sub.0) are coded bits. Also, at the 32-tuple QAM data, uncoded bits (Y.sub.4 Y.sub.3 Y.sub.2) are mapped so as to be 90.degree. rotationally invariant, and the coded bits (Y.sub.1 Y.sub.0) are mapped so as to have complimentary relationship at a time of 180.degree. rotation. For example, in case of a codeword "A" of FIG. 2, the value of the uncoded bits "100" corresponds to an identical bit value "100" with respect to .+-.90.degree. rotation, while the value of the codeword bits "01" corresponds to a bit value "10" with respect to .+-.180.degree. rotation; the bit values having a complementary relationship with each other. Thus, when a phase error of .+-.180.degree. is generated in the Trellis coded 32-tuple QAM data as described above, such an error can be easily detectd.
However, such a differential coding/decoding method cannot find sync data in a demodulator when a phase error of .+-.90.degree. is generated. Accordingly, correct decoding cannot be performed.